A typical liquid crystal display (LCD) device has the advantages of portability, low power consumption, and low radiation, and has been widely used in various portable information products such as notebooks, personal digital assistants (PDAs), video cameras and the like. Furthermore, the LCD is considered by many to have the potential to completely replace CRT (cathode ray tube) monitors and televisions.
One kind of conventional LCD device includes an LCD panel, a backlight source for illuminating the LCD panel, and a power supply circuit for driving the LCD panel and the light source.
Referring to FIG. 3, this is a block diagram of a conventional LCD device. The LCD device 1 includes an alternating current (AC) filter circuit 11, a rectifier and voltage step-down circuit 12, an LCD panel 13, a scaler 14, an LED driving circuit 15, and an LED array 16.
The AC filter circuit 11 generally includes capacitors with large capacitance and windings with large inductance, in order to filter surge voltages from an external electrical network, and then output steady AC current to the rectifier and voltage step-down circuit 12. Then the AC current is rectified and reduced to variety of direct current (DC) voltages, such as 3 volts (V), 5V, 9V, and the like. The DC voltages are used for driving the LCD panel 13 and the scaler 14.
The scaler 14 receives video signals from an external device, processes the video signals, and then sends the video signals to the LCD panel 13 for display of corresponding images. At the same time, the scaler 14 outputs a pulse width modulation (PWM) signal to the LED driving circuit 15. The LED driving circuit 15 receives the PWM signal, and correspondingly drives the LED array 16 to emit light beams for illuminating the LCD panel 13.
The AC filter circuit 11 and the rectifier and voltage step-down circuit 12 both include large capacitors and large windings. In operation, electrical power is stored in the capacitors and the windings. However, when the AC power is cut off, electrical power stored in the capacitors and the windings is released. The released electrical power drives the LCD panel 13 and the LED array 16 to continue working for a moment after the AC power is cut off. Because the released power is abnormal and uncontrolled, colors displayed on the LCD panel 13 are correspondingly abnormal and uncontrolled. For example, so-called red and blue noise colors are displayed. The noise colors reduce a display quality and aesthetic appeal of the LCD device 1.
Accordingly, what is needed is an LCD device that can overcome the above-described deficiencies.